Imagine an IP voice and unified communications system that can be integrated into any application and customised to meet business needs. Sounds great, right? Well that project is the Asterisk IP-PBX and it's free to use and you get the source code. A far cry from proprietary PBX systems perhaps, but Asterisk has a vibrant ecosystem and is replacing systems from more established telephony vendors. Following interviews with the leaders of the Horde and Free Telephony projects, the Open Source Identity series talked to Asterisk founder and Digium CTO Mark Spencer about how one application can have such a profound effect on businesses and how open source can be a tough competitive landscape.